A gunman killed 38 holidaymakers - including 30 Britons and three people from Ireland - in last month’s beach massacre at Sousse which came after 22 people, mostly tourists, were killed at the National Bardo Museum outside Tunis in March.

All of the Britons who were killed in Sousse on June 26 were Thomson and First Choice customers.

Following the government’s warning, Thomson and First Choice immediately cancelled all flights to Tunisia for the summer season adding that “as a precautionary measure we have taken the decision to repatriate all British Thomson and First Choice staff currently working in Tunisia within the next 24 hours.”

Thomas Cook, which cancelled all future bookings to Tunisia up to October 31, said it is putting on an extra flight today to bring its customers back to the UK.

The returning holidaymakers will touch down in Manchester as some of the funerals of the Britons killed in Sousse take place.

Foreign Secretary Philip Hammond told the BBC: “While we do not have any information suggesting a specific or imminent threat, since the attack in Sousse the intelligence and threat picture has developed considerably leading us to the view that a further terrorist attack is highly likely.”

In updated travel advice, the FCO said: “Since the attack in Sousse, we have been working closely with the Tunisian authorities to investigate the attack and the wider threat from terrorist groups in Tunisia.

“Although we have had good co-operation from the Tunisian government, including putting in place additional security measures, the intelligence and threat picture has developed considerably, reinforcing our view that a further terrorist attack is highly likely.

“On balance, we do not believe the mitigation measures in place provide adequate protection for British tourists in Tunisia at the present time and we have therefore changed our travel advice accordingly.”

Holidaymakers should contact their tour operators about returning to Britain and independent travellers should make their own arrangements to leave, the FCO said.

A Thomas Cook spokesman said: “The safety and well-being of our customers is our primary concern ... In response to the recent change in FCO advice we have taken this decision to bring all Thomas Cook customers back to the UK as soon as we can using third party carriers and on our 10 scheduled flights over the weekend.

“Thomas Cook is strongly advising its guests in the country to return on these flights. We are also sending our specialist assistance teams to Tunisia to offer customers additional support in resort.”

Monarch Airlines said it is arranging to repatriate all customers in resorts “back to the UK as soon as possible” in response to the changed advice. Flights have been organised to take them out of Tunisia by this weekend.

The airline is cancelling all flights to Enfidha for the rest of the summer season with “immediate effect”. All customers who have already booked to travel there will get a full refund or credit or they may be able to change their destination.

The Association of British Travel Agents (ABTA) urged people due to travel to Tunisia to contact the company they booked through.

It said: “For anyone due to travel, you should be aware that travelling out to Tunisia at this time is likely to invalidate your travel insurance policies. Most insurance policies will still provide cover for travellers in a country at the time of Foreign Office advice change.

“Those with holidays booked within the next 48 hours to Tunisia are advised to contact your travel company to discuss available options. If you have booked a package you will be entitled to a refund or alternative holiday.”

The new advice is another hammer blow for the Tunisian tourist industry.

Nabil Ammar, Tunisian ambassador to the UK, said the British Government’s announcement was what the terrorists wanted.

He told BBC Two’s Newsnight: “This is what the terrorists want. By damaging the tourism, by having foreigners leaving the country, they damage the whole sector and put so many people out of work and on the streets.

“Hotels have to close and this is an important industry.

“One of the sources of terrorism is lack of hope. It is not the only motor of it but it is one of the very important origins.”

In response to the terror attacks Tunisia announced plans to build a barrier along the border with Libya - including fencing, a sand wall, trenches and surveillance posts - to keep out extremists following recent terror attacks claimed by the Islamic State (IS) group.

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